Many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packing material.
A typical example of such a package is the parallelepiped packing container for liquid or pourable food products known by the name of Tetra Brik Aseptic (registered trademark), which is formed by folding and sealing a strip-rolled packing material. The rolled packing material comprises layers of fibrous material, e.g. paper, covered on both sides with heat seal plastic material, e.g. polyethylene.
For aseptic packing of long-storage products, such as UHT milk, the packing material comprises a layer of barrier material, e.g. an aluminium sheet, which is superimposed on the layer of heat seal plastic material and is in turn covered with another layer of heat seal plastic material forming the inner face of the package actually contacting the food product.
As is known, packages of the above type are produced on fully automatic packing machines, on which a continuous tube is formed from the packing material supplied in strip form. The strip of packing material is sterilized on the packing machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution; following sterilization, the sterilizing agent is removed, e.g. vaporized by heating, from the surfaces of the packing material; and the strip of packing material so sterilized is kept in a closed sterile environment, and is folded and sealed longitudinally to form a tube.
The tube is filled with the sterilized or sterile-processed food product, and is gripped at equally spaced cross sections by two pairs of jaws, which operate cyclically and successively on the tube to heat seal the packing material of the tube and form a continuous strip of pillow packs connected to one another by respective transverse sealing bands.
The pillow packs are separated by cutting the respective sealing bands, and are transferred to a final folding station where they are folded mechanically into the final parallelepiped shape.
Packing units of the above type are known in which the reciprocating movement of the jaws is controlled by a cam system comprising an electric motor, the output shaft of which is fitted, for each pair of jaws, with two cams with appropriately differing profiles. By means of respective lever mechanisms, the cams control the up and down movement of a supporting slide to which a respective pair of jaws is hinged, and the reciprocating opening and closing movement of the jaws.
Two facing forming tabs are hinged respectively to the jaws in each pair, and are moved between an open position, into which they are pushed by elastic means, and a closed position in which they mate to define a space defining the shape and volume of the package to be formed in between. The forming tabs are closed by cams fixed to the structure of the unit, and which interact with respective rollers carried by the tabs.
The portion of the tube gripped between each pair of jaws is simultaneously heat sealed transversely by induction heating means on the jaws.
More specifically, one of the jaws in each pair comprises a main body made of nonconducting material, and an inductor housed in a front seat on the main body. The inductor is normally supplied by a high-frequency generator, and comprises a bar made of electrically conducting material and which interacts with the tube material to heat it to the required sealing temperature.
The other jaw in each pair comprises pressure pads made of elastic material and which cooperate with the inductor to heat seal the tube along a respective sealing band.
Once the sealing operation is completed, a knife on one of the jaws in the pair interacting with the tube of packing material is activated to cut the tube along the center of the sealing band and so detach a pillow pack from the bottom end of the tube of packing material. As the bottom end is sealed transversely, the jaws, on reaching the bottom dead center position, can be opened to prevent interfering with the upper portion of the tube. At the same time, the other pair of jaws, operating in exactly the same way, moves down from the top dead center position to repeat the above gripping/forming, sealing and cutting operations.
As stated, the tube of packing material is heat sealed by inducing electric loss current in the aluminium sheet, which current melts the plastic heat seal material locally when the respective pair of jaws grips the tube, so as to seal a cross section of the tube by heat sealing the plastic coating.
From analysis of the packing material during the heat seal operation, the loss current induced in the cross section of the tube of packing material gripped between each pair of jaws has been found to follow a closed path, which is linear along the two longitudinal sides of the region in which the inductor interacts with the tube, and is roughly semicircular close to the edges of the cross section. That is, the current flows linearly in opposite directions along the two longitudinal sides of the region in which the inductor interacts with the tube, and is deflected towards the center of the cross section gripped between the jaws close to the edges of the cross section (“bending-off effect”), so that the sealing band is narrower at the ends than at the central portion. On the other hand, when packing pourable food products containing small solid particles, such as seeds in tomato products, which may get trapped between the nonsealed portions of the two contacting sheets of packing material, the sealing band should be as wide as possible to reduce the likelihood of channels forming through the sealed portion.